The present invention generally relates to mobile radio communication and, more particularly, to methods and systems in which a base station's transmission of filler data is rendered more flexible.
Many mobile radio communication systems of various kinds are known and in use. In all of these systems, the frequency band(s) available for connections limits the number of possible simultaneous connections, or capacity, of the system. Two base stations or mobile stations transmitting on the same radio frequency of a frequency division multiple access (FDMA) system or on the same time slot of the same radio frequency in a time division multiple access (TDMA) system may interfere with each other. This kind of interference is sometimes called co-channel interference because the interference comes from the same radio channel. If the signal strength of the signals relating to one of the connections is not sufficiently strong relative to the strength of the interfering signals, the information on the first connection will then be more or less unintelligible. If the interfering mobiles or base stations are sufficiently distant from each other, however, the signals relating to the connection will be sufficiently stronger than the interference signals and the information of the connections will be received and properly decoded.
In order to be able to use the same radio channel in FDMA systems, or the same time slot of a radio channel in TDMA systems, for more than one connection, some mobile radio systems are set up as cellular systems. The geographical area to be covered by such a system is divided into smaller areas, called cells, and mobiles in a cell communicate with a base station for that cell. Some or all of the available radio channels are distributed among the cells according to a frequency plan.
A conventional frequency plan provides that different radio channels are allotted to a cluster of adjacent or neighboring cells. No two cells in the same cluster can use the same radio channel at the same time. However, cells in different clusters may use the same radio channels. Thus there can be simultaneous multiple use of a radio channel. Such multiple use is sometimes called channel or frequency re-use. The distance between cells using the same radio channel is known as the re-use distance. The re-use distance is selected so that co-channel interference is kept within tolerable levels. In conventional FDMA or TDMA systems where the same radio channel is used throughout a connection, any co-channel interference will last as long as both the connections last when the transmissions occur more or less simultaneously on the same radio channel. Thus, a worst case situation must be considered in frequency planning and cell cluster design to ensure that the minimum acceptable signal quality is maintained.
Frequency hopping is a technique for ensuring that worst case interference scenarios do not prevail for longer than one frequency hop interval as opposed to the duration of the entire connection, this characteristic is commonly being known as interferer diversity. Frequency hopping also provides frequency diversity which combats fading for slowly moving mobile stations. The European GSM standard describes such a frequency hopping system, which is based on a combination of time division multiple access (TDMA) in which a 4.6 ms time cycle on each frequency channel is divided into eight, 560 .mu.s time slots occupied by different users, and frequency hopping in which the frequencies of each of the eight time slots are independent of one another and change every 4.6 ms.
There are, however, several difficulties associated with frequency hopping systems, in general, and the GSM implementation in particular. For example, frequency hopping in GSM has historically been implemented by selecting a different transmitter for each time slot, each transmitter transmitting on a different, fixed carrier frequency. This type of system will now be described with respect to an exemplary base station illustrated in FIG. 1. Therein, each channel has associated therewith signal processing circuitry denoted by blocks 1-5. Although only five signal processing blocks are illustrated for clarity of the figure, those skilled in the art will appreciate that a typical base station will include many more such circuits.
The base station of FIG. 1 also has a plurality of transmitters 6-10. These transmitters are each configured to transmit on a single carrier frequency and receive data for transmission from various ones of the signal processing units 1-5. This data is conveyed over the baseband switch bus 11. Each of the transmitters 6-10 has a dedicated line in the bus to which each of the signal processing units 1-5 can be independently connected via switches such as the switching junction indicated by reference numeral 12.
As one can imagine, the switching system depicted by bus 11 and switches 12 is rather complex and requires a lot of wiring particularly when considering the actual number of signal processing units and transmitters in each base station as compared with the five exemplary branches illustrated in FIG 1. A partial solution to this switching system complexity is to allow each of the transmitters 6-10 to transmit on different frequencies as controlled by the data received at the transmitters from the signal processors. However, this solution fails to address another difficulty related to how conventional systems handle control channels.
Control channels support system functions such as synchronization, broadcast of system information, call set-up, etc. In GSM, the control channels are also used by the mobiles to make signal strength measurements, which information is used to identify a base station for initial access and to determine an appropriate base station candidate for handover. For example, the mobile stations can use the idle time between active slots for measurements on the control channels of adjacent cells' base stations. Since only a few time slots are available for such measurements, the base stations are required to transmit with constant output power on all time slots of the downlink frequency used for their respective control channels.
FIG. 2 illustrates the multiplexing of a logical control channel combination onto time slot 0 of carrier C0 used as the downlink control channel frequency. Each of the other time slots 1-7 can be used to carry voice or data traffic. Therein, `F` stands for the frequency correction channel (FCCH), `S` stands for the synchronization channel (SCH), `B` represents the broadcast control channel (BCCH), `C` denotes a common control channel (CCCH) which includes a paging or access grant channel, and `I` stands for idle. At certain times, however, there is no meaningful information to be transmitted on the control channel. For example, CCCH may have no paging messages to be sent. Moreover, the other time slots 1-7 may have no voice or data traffic to support. At these times, GSM provides for dummy bursts of filler data to be transmitted on otherwise empty time slots to satisfy the requirement that each time slot on the control channel frequency C0 be transmitted on at full power.
Conventionally, this has been accomplished by dedicating a transmitter, for example transmitter 7 in FIG. 1, to the control channel frequency. This allows the system to readily determine when a dummy burst needs to be inserted for time slots to be transmitted by that transmitter. The problem with this solution is that if the filler transmitter 7 malfunctions, then the control channel is lost and no traffic can be handled in that cell until a new transmitter is configured to handle the control channel.